1. Field of the Invention
This invention relates to an electric power steering apparatus in which the steering force of a steering wheel is assisted by an electric motor.
2. Description of the Related Art
FIG. 1 is a schematic sectional view illustrating a prior art electric power steering apparatus. In the apparatus, an upper column shaft 2A to which a steering wheel 1 is attached at the upper end is rotatably supported by a column housing 3. The lower end of the upper column shaft 2A is connected via a torsion bar 4 to the upper end of a lower column shaft 2B which is coupled through a pinion 5a to a rack 5 of the steering gear. A torque sensor 6 which detects a torque acting on the upper column shaft 2A is disposed at the middle portion of the torsion bar 4. A three-phase brushless direct drive electric motor DM is disposed at the upper portion of the lower column shaft 2B. The electric motor DM including a rotor 7 fixed to the lower column shaft 2B, and a stator 8 surrounding the rotor 7. The stator 8 is fixed to the inside of a motor case 9 which is attached to the column housing 3. The output of the torque sensor 6 is derived through a slip ring 11 which is provided in the column housing 3 above the motor case 9.
FIG. 2 is a circuit diagram illustrating a conduction control circuit for the electric motor DM. A series circuit of power MOSFETs (hereinafter, referred to as "power transistors") T1 and T4 is connected between a DC power source +V and the ground. To this series circuit, two series circuits respectively consisting of power transistors T2 and T5 and power transistors T3 and T6 are connected in parallel. The junction point A of the power transistors T1 and T4 is coupled to one end of the U-phase coil 8au of a coil 8a of the electric motor DM, the junction point B of the power transistors T2 and T5 to one end of the V-phase coil 8av of the coil 8a, and the junction point C of the power transistors T3 and T6 to one end of the W-phase coil 8aw of the coil 8a. The other ends of the coils 8au, 8av and 8aw are commonly connected to each other. PWM signals for commutation-controlling the power transistors T1-T6 are supplied from a control unit (not shown) to the gates G1-G6 of the power transistors T1-T6.
The operation of the electric power steering apparatus will be described.
When the steering wheel 1 is turned, this turning operation causes a torque to act on the upper column shaft 2A, and this torque is detected by the torque sensor 6. A torque signal corresponding to the torque which is detected by the torque sensor 6 is supplied to the control unit (not shown). In accordance with this torque signal, the control unit supplies signals for commutation-controlling the power transistors T1-T6 to the gates G1-G6, thereby commutation-controlling the power transistors T1-T6. The currents flowing through the phase coils 8au, 8av and 8aw are controlled by adjusting the conduction angles of the power transistors T1-T6, so as to control the rotating force of the electric motor DM. The rotation of the thus-driven electric motor DM causes the lower column shaft 2B to rotate, thereby assisting the steering force.
This assistance to the steering force causes the acting torque to decrease. When the torque signal of the torque sensor 6 is lowered to the dead zone, it becomes unnecessary to assist the steering force, and therefore the driving operation of the electric motor DM is stopped. In this case, even if the current supply to the phase coils 8au, 8av and 8aw of the stator 8 is shut, off, the rotor 7 continues rotations owing to the inertia. Therefore, the power transistors T1, T2 and T3 are turned off and the power transistors T4, T5 and T6 are turned on, so that the one ends of the phase coils 8au, 8av and 8aw are connected to each other or the phase coils 8au, 8av and 8aw are short-circuited as shown in FIG. 3, and the rotor 7 is braked to stop the rotation by a short circuit current caused by this short circuit.
Alternatively, the power transistors T1 and T4 (T2 and T5, and T3 and T6) may be driven complementarily at a duty ratio of 50%. Also in this case, the short-circuit braking can be conducted in the same manner as described above.
When the phase coils of the electric motor are short-circuited as described above, however, a counter electromotive force is generated in each of the phase coils during a short period of time which continues until the rotor stops, whereby a reverse rotating force acts on the rotor, with the result that a rotating force which is opposite in direction to the rotating force assisting the steering force and applied by the electric motor acts on the steering wheel. This produces disadvantages that a sense of incongruity is produced in the steering and that the return of the steering wheel 1 to a straight ahead position for the vehicle wheels can not be easily done.
When the steering wheel 1 is to be returned to the straight ahead position, the apparatus receives a returning force from the wheels of the automobile, and the steering wheel 1 applies a force opposing to the returning force, so that the direction of the current for driving the electric motor in the direction in accordance with the detected torque is opposite to that of the current of the counter electromotive force due to the rotating force applied from the wheels of the automobile to the electric motor.
In order to comply with this and also to detect a winding layer short or the like in the electric motor, currents flowing through the bridge portions of the power transistor T1 and T2, etc. are detected, and the detected currents are reflected in conduction control of the power transistor T1 and T2. In this system, however, the detection of currents for three phases requires the provision of detection resistors in bridge portions for at least two phases. This hinders the electric motor from being miniaturized and made lighter. In order to reduce the number of required resistors, a system has been proposed by the applicant (U.S. patent application Ser. No. 942,636). In this proposed system, as shown in FIG. 4, a resistor R is disposed in the DC circuit portion to detect a current flowing through this portion, and the detected current is used in a feedback control. However, the proposed system cannot naturally detect a circulating current caused by a short-circuit as described above. When the electric motor is driven by the circulating current, there arises a problem in that the steering sense is impaired.